A hidden hinge is structured to prevent its external appearance from being exposed to the outside when a door is closed. Such a hidden hinge is favorable to crime prevention since the door cannot be separated from a doorframe with the door closed and is in wide use for a fire door, an entrance door, a furniture door or the like since it does not spoil the beauty of the door.
In particular, since the hidden hinge cannot be broken up once being built, unlike an existing pivot hinge or butterfly hinge which can be broken up by a simple tool, it has a high advantage in the aspect of safety for crime prevention.
FIG. 1 shows a representative example of a conventional hidden hinge which has been in wide use for a fire door, including, as main components, a fixing piece 1, an axial pin 2 and a rotating piece 3. The fixing piece 1 has flanges 1a and 1b which are of a ‘’-like clamp and are vertically formed in vertical linear ends. The axial pin 2 serves as a rotating supporting axis and is vertically disposed at one side of the fixing piece 1. The rotating piece 3 has a ‘’-bent structure and can be freely rotated within a predetermined range of angle under a state where it is assembled into the fixing piece 1 by the axial pin 2 by providing a connecting piece 3b of the axial pin 15, in which the axial pin 15 is inserted, in one bent surface 3a of the rotating piece 3 to be assembled with the fixing piece 1.
On the other hand, screw holes 1c and 1d are formed in flanges 1a and 1b of the fixing piece 1, respectively, and the fixing piece 1 is fastened, by a screw 5, to a fixing piece fastener 4 for fixing the fixing piece 1 to a doorframe. In addition, several screw holes 3d are formed in the other bent surface 3c of the rotating piece 3 and a door leaf connecting fastener 6 for connecting the door (for example, a fire door) to the rotating piece 3 is fastened, by screws 7, to the screw holes 3d. Reference numeral 8 denotes an auxiliary fastener of the door leaf connecting fastener 6 which is welded with the door and is coupled to the door leaf connecting fastener 6 by a screw 9.
FIG. 2 shows an application of the above-configured hidden hinge to a fire door. Referring to FIG. 2, a structure where the hidden hinge is installed in a door leaf D and a doorframe F constituting the fire door is as follows. That is, a hinge installation part having a square hole through which a front of the hidden hinge is exposed is provided in a corresponding portion of the doorframe F in which the hidden hinge is installed, and the fixing piece 1 of the hidden hinge is fixed to the hinge installation part.
However, since the fixing piece fastener 4 is welded to the inner side of the doorframe F, with the fixing piece 1 arranged in the hinge installation part of the doorframe F, after the fixing piece fastener 4 is fastened to the flanges 1a and 1b of the fixing piece 1 by the screw 5, there is a problem of difficulty in later replacement of the fixing piece 1.
In other words, the door leaf connecting fastener 6 is coupled to the auxiliary fastener 8 and the auxiliary fastener 8 is welded to the door leaf D, and subsequently, the door leaf D is combined to the doorframe F when the door leaf connecting fastener 6 is fastened to the rotating piece 3 by the screws 7.
In construction of using the conventional hidden hinge to install the door leaf D in the doorframe F in this manner, the door leaf connecting fastener 6 has to be fastened to the rotating piece 3 of the hidden hinge installed in the hinge installation part of the doorframe F, with the door leaf connecting fastener 6 coordinated with the screw holes, under a state where the heavy door leaf D is lifted by a required height. This requires high work precision and long working time and makes the hidden hinge difficult to be fixed to a desired position due to weight of the door, which may result in poor construction ability. In addition, the screws 7 have to be unscrewed one by one when the door leaf D needs to be separated from the doorframe F or needs to be adjusted and the door may be drooped due to weight of the door, which may result in poor workability.
In addition, when the door leaf D is installed in the doorframe F with the conventional hidden hinge, the vertical height and horizontal interval of the door leaf D have to be adjusted precisely so that the door leaf D can fit into the doorframe F. However, it is very difficult to adjust the height of the door leaf D with respect to the doorframe F under the state where the door leaf connecting fastener 6 to be assembled in the rotating piece 3 of the hidden hinge and the auxiliary fastener 8 are installed in the door leaf D, which may result in great difficulty in construction. Moreover, since the fixing fastener 4 is welded to the doorframe F after the fixing piece 1 of the hinge is previously inserted in a punching back surface of the doorframe in construction of the doorframe, there arises a problem of difficulty in replacement of the hinge in the event of later failure or aging of the hinge.